Visual Hallucinations in Schizophrenia

Post by Anastasia Sares

What’s the science?

Sensory disturbances and hallucinations are common occurrences for people with psychotic disorders, including schizophrenia and schizotypal disorders. Most research on hallucinations has focused on auditory hallucinations (like hearing voices), since these are more common. But visual hallucinations are also possible—and we may have been underestimating their prevalence. This week in Frontiers in Psychiatry, Silverstein, and colleagues reviewed the research on visual hallucinations in schizophrenia, emphasizing how they involve multiple brain systems.

What do we already know?

Visual impairments are well documented in schizophrenia. They range from so-called “low-level” problems, like sensitivity to changes in light levels, all the way to “high-level” problems, like difficulties with face or object recognition. Distortions of facial features, object size and location, or even temporary partial blindness, are also possible. Estimates of the prevalence of full-blown visual hallucinations in schizophrenia vary wildly (from 4% to 65%, depending on the study). According to Silverstein and Lai, this could be a result of unstandardized clinical evaluations. If clinicians don’t ask specifically about visual hallucinations, they could go under-reported. Moreover, there is a disagreement about the nature of these hallucinations: whether they are predominantly rich and detailed, involving people and 3D objects well-integrated with the environment, or whether they are more likely to be simple geometric shapes or lights.

A sensory disorder

There are a number of proposed explanations for visual hallucinations in schizophrenia, but most have to do with a problem in the brain’s sensory processing. Here are a few:

1.     A “low-level” problem with vision, like degeneration of the retina, prevents people from getting information about the outside world, forcing their brain to fill in the blanks;

2. A deficiency of the neurotransmitter acetylcholine in some brain areas leads to “fuzzy” sensory input that is easily misinterpreted—at the same time, an excess of acetylcholine in other areas could cause mental associations and imagery to be experienced as external stimuli.

3.     Cellular and network changes in regions like the hippocampus (a memory structure) and the default mode network (a rumination and self-reflection network), can conspire to make remembered or imagined events intrude and replace incoming sensory information.

Many of these ideas have simultaneously gained support over the years, including (maybe surprisingly), simple degeneration of the neural tissue in the eye. These sensory anomalies could combine with a person’s own fears or ideations, leading to false explanations for the unusual sensations; this would explain how the delusions characteristic of schizophrenia come about.

What’s new?

Silverstein and Lai suggest that these various mechanisms might produce disturbances or hallucinations that differ in character: low-level deficits in the eye might only produce visual disturbances, while differences in the default mode network or hippocampus could result in rich and detailed hallucinations. However, they stress that future researchers will have to work hard to determine the chain of causality from the brain to behavior—currently, we only have correlational evidence.

What's the bottom line?

Schizophrenia was, and sometimes still is, stigmatized as a result of hallucinations. Viewing it as a sensory disorder can help us to understand and approach it with a little more compassion. Further, understanding the mechanisms underlying hallucinations and how they function at a systemic level helps to provide a deeper understanding of this complex condition.

Silverstein & Lai. The Phenomenology and Neurobiology of Visual Distortions and Hallucinations in Schizophrenia: An Update. Frontiers in Psychiatry (2021). Access the original scientific publication here.